A tuning fork crystal oscillator used for a vibratory gyro, etc., is manufactured by the steps including cutting a crystal oscillator piece of a desired shape from a crystal wafer, forming electrodes for causing the crystal oscillator piece to oscillate, and packaging the crystal oscillator piece with the electrodes formed thereon into a container. In particular, the step of cutting the crystal oscillator piece from a crystal wafer is an important step because the shape of the crystal oscillator piece determines the frequency of vibration and greatly affects the device performance.
FIG. 15 is a diagram showing the crystallographic axes of a crystal oscillator piece.
As shown in FIG. 15, a crystal wafer is fabricated using, for example, a Z-piece obtained by cutting a crystal along a plane perpendicular to the Z axis of the crystal or a crystal wafer 100 obtained by rotating the Z-piece about the X axis by an angle of 0° to 15°. The crystallographic axes, after rotation about the X axis, are X, Y′, and Z′. This means that the principal plane of the crystal wafer 100 is the X-Y′ plane.
FIG. 16 is a diagram schematically showing the crystal oscillator piece 110 cut from the crystal wafer 100.
FIG. 16(a) is a schematic plan view of the crystal oscillator piece 110, FIG. 16(b) is a diagram showing one example of a cross-sectional view taken along line A-A′ in FIG. 16(a), and FIG. 16(c) is a diagram showing another example of a cross-sectional view taken along line A-A′ in FIG. 16(a).
The crystal oscillator piece 110 is made up of a support portion 111, a base portion 112, and vibrating tines 113. The vibrating tines 113 are the portions that vibrate. The vibrating tines 113 each have a width in the X axis direction, a length in the Y′ axis direction, and a thickness in the Z′ axis direction.
In the step of cutting the crystal oscillator piece 110 from the crystal wafer 100, a method utilizing photolithography and wet etching is employed since it can mass-produce small-sized crystal oscillator pieces with good accuracy and at low cost.
FIG. 17 is a diagram showing a method for manufacturing the crystal oscillator piece. FIG. 17 shows cross sections of the vibrating tines of the crystal oscillator piece.
First, as shown in FIG. 17(b), corrosion resistant metal films 200a and 200b having resistance to crystal etchants and photoresist films 201a and 201b overlying the respective corrosion resistant metal films 200a and 200b are formed on the upper and lower surfaces of the crystal wafer 100 prepared, as shown in FIG. 17(a), to a desired thickness.
Next, as shown in FIG. 17(c), the photoresist films 201a and 201b are exposed to radiation through two photomasks 205 and 206, respectively, that have oscillator patterns written thereon that perfectly overlay each other when placed facing each other.
Next, the photoresist films 201a and 201b are developed. Then, using the thus developed photoresist patterns as masks, the corrosion resistant metal films 200a and 200b are patterned, as shown in FIG. 17(d), thus forming etching masks 207a and 207b for crystal etching.
Next, the remaining photoresist films are removed. Thereafter, the crystal wafer 101 with the etching masks 207a and 207b formed on both surfaces thereof is immersed in an etching solution of hydrofluoric acid, and the portions of the crystal that are not covered with the etching masks 207a and 207b are dissolved from both surfaces, as shown in FIG. 17(e). After that, the etching masks 207a and 207b are removed, completing the fabrication of the crystal oscillator piece 110 such as shown in FIG. 16.
In another known method for manufacturing a crystal oscillator piece, an etching mask is patterned only on one surface, with the other surface completely covered with a corrosion resistant metal film, and the one surface is etched (for example, refer to patent document 1).
In still another known method for manufacturing a crystal oscillator piece, the etching mask pattern 207d on the lower surface is formed wider than the pattern 207c on the upper surface, as shown in FIG. 18, and etching is performed using the upper surface pattern 207c as the reference pattern (for example, refer to patent document 2).
FIG. 19 is a diagram explaining the direction of vibration of the crystal oscillator piece when it is used for a vibratory gyro.
FIG. 19(a) is a perspective view of the crystal oscillator piece of FIG. 16(a), FIG. 19(b) is a diagram showing one example of the direction of vibration in an A-A′ cross section of FIG. 19(a), and FIG. 19(c) is a diagram showing another example of the direction of vibration in the A-A′ cross section of FIG. 19(a).
As shown in FIG. 19(a), when using the tuning fork crystal oscillator for a vibratory gyro, flexural vibration in the X axis direction is used as driving vibration, and flexural vibration in the Z′ axis direction is used as detection vibration which occurs when an angular velocity is applied. In this arrangement, in the absence of an applied angular velocity, the vibration in the Z′ axis direction should not normally occur, as shown in FIG. 19(b).
However, in the tuning fork crystal oscillator manufactured by the prior art manufacturing method, there have been cases where a vibration component in the Z′ axis direction is observed, as shown in FIG. 19(c), when actually no angular velocity is applied. The vibration component in the Z′ axis direction arising from this oblique vibration is called the leakage vibration; since this vibration is indistinguishable from the detection vibration, there has been the problem that the S/N and temperature characteristics of the gyro degrade due to the leakage vibration.
In the case of tuning fork crystal oscillators for ordinary applications, the tuning fork vibration is likewise produced by using the flexural vibration in the X axis direction, and in this case also, there has been the problem that the oblique vibration containing the Z′ direction component causes the crystal impedance to rise, leading to a degradation of the characteristics.
It is believed that crystal residual portions generated when fabricating the crystal oscillator piece by etching have some bearing on the oblique vibration. Since the crystal has etching anisotropy, the etch rate is different in different directions of the crystal. As a result, the side faces of the vibrating tines 113 of the crystal oscillator piece after etching are not perpendicular to the principal plane, but residual portions are left thereon forming angles.
For example, as shown in FIGS. 16(b) and 16(c), the cross-sectional shape of the crystal oscillator piece 110 is not precisely rectangular, but residual triangles or other shapes are formed on the +X and −X side faces in the Y′-Z′ plane. FIG. 16(b) shows the cross-sectional shape after etching for a short period of time, and FIG. 16(c) shows the cross-sectional shape after etching for a long period of time.
If such residual portions are present, the driving vibration which should normally occur only in the X axis direction, as shown in FIG. 19(b), may be disrupted, depending on how the residual portions are formed, and the vibration occurs in the oblique direction by being accompanied by a component vibrating in the Z′ axis direction, as shown in FIG. 19(c). This results in the generation of leakage vibration. The leakage vibration arising from such oblique vibration tends to occur rather often when the crystal oscillator piece is manufactured by the prior art method. There has therefore developed a need to suppress the oblique vibration and reduce the leakage vibration.
There is also a document that analyzes the relationship between the oblique vibration and the directions of the principal axes of the cross section of a crystal oscillator piece (for example, refer to non-patent document 1).
Further, it is known in the prior art to provide a piezoelectric device which is fabricated by forming grooves in the vibrating tines of a tuning fork crystal oscillator piece, with each groove (electric field forming groove) provided with electrodes for generating a prescribed electric field, thereby causing each vibrating tine to repeatedly generate flexural motion.
Patent document 1: Japanese Unexamined Patent Publication No. S52-035592 (Page 3, FIG. 3)
Patent document 2: Japanese Unexamined Patent Publication No. 2006-217497 (Page 5, FIG. 1)
Patent document 3: Japanese Unexamined Patent Publication No. 2004-007428 (FIG. 2)
Non-patent document 1: Motohiro FUJIYOSHI et al., IEICE Transactions, C Vol. J87-C, No. 9, p. 712